Characterization of the zebrafish cx36.7 gene promoter: Its regulation of cardiac-specific expression and skeletal muscle-specific repression.

Zebrafish connexin 36.7 (cx36.7/ecx) has been identified as a key molecule in the early stages of heart development in this species. A defect in cx36.7 causes severe heart malformation due to the downregulation of nkx2.5 expression, a result which resembles congenital heart disease in humans. It has been shown that cx36.7 is expressed specifically in early developing heart cardiomyocytes. However, the regulatory mechanism for the cardiac-restricted expression of cx36.7 remains to be elucidated. In this study we isolated the 5'-flanking promoter region of the cx36.7 gene and characterized its promoter activity in zebrafish embryos. Deletion analysis showed that a 316-bp upstream region is essential for cardiac-restricted expression. This region contains four GATA elements, the proximal two of which are responsible for promoter activation in the embryonic heart and serve as binding sites for gata4. When gata4, gata5 and gata6 were simultaneously knocked down, the promoter activity was significantly decreased. Moreover, the deletion of the region between -316 and -133bp led to EGFP expression in the embryonic trunk muscle. The distal two GATA and A/T-rich elements in this region act as repressors of promoter activity in skeletal muscle. These results suggest that cx36.7 expression is directed by cardiac promoter activation via the two proximal GATA elements as well as by skeletal muscle-specific promoter repression via the two distal GATA elements.